Method of forming and treating a metal film, semiconductor device and wiring board

ABSTRACT

A method of forming a metal film of copper on the surface of a resin substrate by successively effecting the conditioning treatment, Pd activation treatment, electroless copper plating treatment and electrolytic copper plating treatment. In the conditioning treatment, a conditioning treatment solution is interposed in the form of a thin layer between the surface and a cover glass plate, and the surface is irradiated with ultraviolet light from the upper side of the cover glass plate. Irradiation with ultraviolet light in the presence of the treatment solution causes the molecules in the surface of the resin to chemically react with the component of the treatment solution, whereby the surface of the resin is more activated to improve the adhesion with copper that is deposited on the surface of the resin.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of forming and treating a metal film on a resin substrate, to a semiconductor device having a metal film formed on an insulating resin layer and to a wiring substrate. More particularly, the invention relates to a method of forming and treating a metal film which is formed by plating on the surface of the insulating resin substrate maintaining an increased adhering force, to a semiconductor device and to a wiring board.

[0003] 2. Description of the Related Art

[0004] Conventional methods of forming a metal film have been applied to, for example, build-up substrates, wiring boards inclusive of a semiconductor package using the above substrate as well as to semiconductor devices called chip-size packages forming a wiring pattern on a semiconductor element via an insulating resin layer.

[0005] When, for example, a wiring pattern is formed on a semiconductor device, a wiring layer is laminated between the insulating resin layers by using, as an underlying member, a substrate having electric insulation, such as polyimide or epoxy resin, applying a coating thereon, or press-adhering an electrically insulating resin film thereon to form an insulating resin layer and, then, forming an electrically conducting layer by plating or the like on the surface of the insulating resin layer. The electrically conducting layer formed on the surface of the insulating resin layer is formed into a predetermined pattern by etching thereby to form a wiring pattern on the surface of the insulating resin layer. Or, the insulating resin layer is masked in a predetermined pattern and, then, an electrically conducting layer is formed thereon by plating to form a wiring pattern.

[0006] When an electrically conducting layer is to be formed by plating on the surface of the insulating resin layer, however, it is a practice to effect the plating after the surface of the insulating resin layer is subjected to the roughening treatment (desmear treatment) in order to enhance the adhesion between the electrically conducting layer that is formed and the insulating resin layer as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-57456. The desmear treatment is conducted by etching the surface of the insulating resin layer by using an etching solution such as potassium permanganate or sodium permanganate.

[0007] In this case, the conductor is filled in the recessed portions in the surface of the insulating resin layer that is formed rough as the surface of the insulating resin layer is subjected to the desmear treatment by using an etching solution; i.e., the conductor of the wiring pattern is intimately adhered to the insulating resin layer due to the anchoring action. As the surface of the insulating resin layer becomes increasingly rough, however, precision of the pattern is affected by the ruggedness of the surface at the time of forming the wiring pattern by etching the conducting layer, causing a problem in that it becomes difficult to precisely form very fine wiring patterns.

[0008] In forming the wiring pattern by etching the conducting layer depending upon the degree of roughness of the surface of the insulating resin layer, the biting amount increases with an increase in the roughness of the surface of the insulating resin layer, and the biting amount decreases with a decrease in the roughness of the surface. That is, when the surface of the insulating resin layer is very rough, the etching solution easily enters onto the side surfaces of the wiring pattern through the rough portions at the time of forming the wiring pattern by etching the conducting layer, and the side surfaces of the wiring pattern assume a floated state. When the surface is very rough, therefore, it becomes difficult to finely form the wiring pattern.

[0009] When the surface of the insulating resin layer becomes increasingly rough, further, there occurs a problem of an increase in the transmission loss of high-frequency signals. The transmission loss decreases with a decrease in the roughness of the surface. As the surface roughness of the insulating resin layer increases, further, the resistance against migration decreases. It is therefore desired that the surface roughness of the insulating resin layer on which the conducting layer is formed is as small as possible. In forming the conducting layer on the insulating resin layer, therefore, it is desired to decrease the surface roughness of the insulating resin layer as much as possible and to improve the adhesion between the insulating resin layer and the conducting layer.

[0010] When the conducting layer is to be formed on the insulating resin layer in the semiconductor devices, in general, a metal film is formed, for example, by electroless copper plating and by subsequent electrolytic copper plating to thereby form a wiring substrate. The copper plating, however, adheres less to the resin substrate than does the nickel plating. When a metal film that serves as a conducting layer is to be formed by the copper plating, therefore, a more reliable adhesion is required between the metal film and the resin substrate.

[0011] In order to improve the adhesion, therefore, there have been proposed a variety of methods of reforming the surface of the resin substrate and, then, effecting the electroless plating on the surface thereof as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-87964. For example, the surface of the resin substrate is irradiated with ultraviolet laser, the resin substrate being placed in an atmosphere of an amine compound gas or an amide compound gas, and, then, the electroless plating is effected.

[0012] As the pretreatment for effecting the electroless plating onto the resin substrate, there have further been proposed a method of irradiating the surface of the resin substrate with ultraviolet light and, then, effecting the electroless plating on the surface of the resin substrate as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-253869, and a method of improving the adhesion by conducting a step of surface treatment by bringing the surface into contact with an alkali solution containing a non-ionic surfactant having a polyoxyethylene bond as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-88361.

[0013] There has further been proposed a method of improving the adhesion of the metal film formed on the resin substrate by electroless plating by reforming the surface of the resin substrate by the irradiation with ultraviolet light so as to absorb a silane coupling agent having an amino functional group and by promoting the imparting of a tin-palladium catalyst as taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-310873.

[0014] In addition to the method of reforming the surface of the resin substrate by the irradiation with ultraviolet light, there has further been proposed a method of improving the adhesion by treating the surface of the resin substrate with a plasma and with ultraviolet light in this order, effecting the electroless plating to form a functional group that produces the adhesion to the metal film formed by the electroless copper plating, and decreasing the surface roughness of the resin substrate, as taught in Japanese Unexamined Patent Publication (Kokai) No. 2002-57456 described above.

[0015] When a metal film that serves as a conducting layer is to be formed on a resin substrate which is an insulating resin layer used for a semiconductor device, there have been proposed a variety of methods of improving the adhesion between the resin substrate and the metal film in order to increase the peeling strength that represents the adhesion to a practicable level.

[0016] However, the above methods are based on a prerequisite of desmearing the surface of the resin substrate by etching. The etching treatment is generally conducted by immersing the resin substrate in a strongly acidic etching solution such as a mixed solution of chromic acid and sulfuric acid, a mixed solution of dichromic acid and sulfuric acid, chloric acid, or a mixed solution of sulfuric acid and perchloric acid. However, the etching solution is a chemical solution which is dangerous and is highly likely to cause a public hazard, and must be handled and drained with a sufficient degree of attention accounting for a large burden in the step of plating for forming a metal film.

[0017] According to the above methods, further, even though the adhesion can be improved between the resin substrate and the metal film, it is necessary not only to irradiate the ultraviolet light but also to prepare a treating agent for promoting the reformation of the surface of the resin substrate after the irradiation in addition to preparing the treatment solutions in the ordinary steps of treatment, causing such problems as an increased number of the treatment steps and an increased cost of treatment.

[0018] In improving the surface of the resin substrate, further, when a plasma treatment is to be effected following the irradiation with ultraviolet light, it is necessary to provide a plasma treatment apparatus in addition to ultraviolet light irradiation apparatus. Provision of the plasma treatment apparatus causes an increase in the cost of facility making it difficult to provide the products inexpensively.

SUMMARY OF THE INVENTION.

[0019] It is therefore an object of the present invention to provide a method of forming and treating a metal film capable of easily improving the adhesion between the resin substrate and the metal film simply by using a treatment solution used in the conventional treating steps of forming a metal film on the surface of a resin substrate such as of a semiconductor device, and executing the following step of treatment while the effect of irradiation with ultraviolet light lasts, as well as to provide a semiconductor device having, formed on the resin substrate, a metal film relying upon the above method of forming and treating a metal film, and a circuit wiring board.

[0020] In order to solve the above problems, the present invention provides a method of forming and treating a metal film on the surface of a resin substrate by conducting the treatments according to a step of forming plating nuclei and a step of electroless plating, or by conducting the treatment according to a step of electrolytic plating after the step of electroless plating, wherein the surface is irradiated with ultraviolet light through a treatment solution that is used prior to the step of electroless plating.

[0021] The treatment solution is a conditioning treatment solution for pre-treating the surface prior to the step of forming plating nuclei, or is a catalyzing treatment solution applied to the surface in the step of forming the plating nuclei. Ultraviolet light is irradiated through the conditioning treatment solution or through the catalyzing treatment solution.

[0022] Further, the conditioning treatment solution or the catalyzing treatment solution is interposed in the form of a thin layer between the glass plate and the surface.

[0023] The invention further provides a semiconductor device having a metal film formed on an insulating resin layer by the above method of forming and treating the metal film, and a wiring board having a metal film formed on the resin substrate by the above method of forming and treating the metal film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other features, objects and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the drawings in which like reference characters designate like or corresponding parts throughout several views, and in which:

[0025]FIG. 1 is a flowchart illustrating the procedure of a processing of a method of forming and treating a metal film according to a first embodiment;

[0026]FIG. 2 is a diagram illustrating an example for irradiating ultraviolet light in the step of conditioning treatment in the method of forming and treating a metal film according to the first embodiment;

[0027]FIG. 3 is a flowchart illustrating the procedure of a processing of a method of forming and treating a metal film according to a second embodiment;

[0028]FIG. 4 is a diagram illustrating an example for irradiating ultraviolet light in the step of conditioning treatment in the method of forming and treating a metal film according to the second embodiment;

[0029]FIG. 5 is a flowchart illustrating the procedure of a processing of the method of forming and treating a metal film in combination with a further embodiment;

[0030]FIG. 6 is a flowchart illustrating the procedure of a processing of the method of forming a metal film which serves as a basis of the present invention;

[0031]FIGS. 7A to 7E are diagrams illustrating the steps of processing of the method of forming the metal film illustrated in FIG. 6; and

[0032]FIG. 8 is a flowchart illustrating the procedure of a processing of another method of forming a metal film, which serves as a basis of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Embodiments of the method of forming a metal film according to the present invention will now be described with reference to the drawings.

[0034] A plating method has heretofore been employed for forming a metal film on the surface of a resin substrate. When a metal film which is copper is to be formed on the surface of the resin substrate such as of polyimide, for example, the plating method generally comprises a step of pretreatment such as dewaxing the surface of the resin, a step of etching treatment, a step of catalyzing treatment, a step of accelerating treatment, a step of electroless copper plating treatment and a step of electrolytic copper plating treatment.

[0035] As described above, the plating is not directly effected on the surface of the resin substrate but is effected after various treatments are executed because the resin has a hydrophobic property in that it is unlikely to be wetted with water. The metal film cannot be formed on the surface of the resin substrate even if the plating is directly effected on the surface. When the surface treatment is to be effected in the aqueous solution, the surface of the resin substrate must have been rendered to be hydrophilic so that it can be easily wetted with water. In order for the plated metal to be intimately adhered to the surface of the resin substrate, the surface of the resin substrate is rendered hydrophilic, the surface of the resin is activated by forming polar groups thereon, and the surface of the resin is desmeared so as to possess a roughness such as fine pores. This treatment is the etching treatment.

[0036] In order that the plating nuclei are precipitated, further, the surface of the resin must be activated with palladium (Pd) and, hence, is immersed in a catalyzing solution containing PdCl₂ and SnCl₂ so that the catalytic metal is adsorbed by the surface of the resin. This treatment is the catalyzing treatment.

[0037] Upon executing the catalyzing treatment, metal palladium that serves as plating nuclei is precipitated on the surface of the resin in the accelerating solution which contains HCl, H₂SO₄ or NH₄F.HF in the step of accelerating treatment because a complex of Pd and Sn has been adsorbed by the surface of the resin substrate.

[0038] Then, in the step of electroless plating, metal copper is electroless-plated on the surface of the resin by the catalytic action of the plating nuclei that are precipitated on the surface of the resin, and a metal film is formed on the whole surface of the resin. The metal film formed by the electroless plating plays the role of a power feeding layer for the electrolytic plating, and is formed to a thickness of, usually, about 0.5 to about 2.0 μm. Thereafter, the electrolytic copper plating is effected in the step of electrolytic copper plating treatment until a predetermined thickness, that can be used for the wiring pattern, is obtained, thereby to form a metal film.

[0039] The above-mentioned steps of treatments constitute a general method of forming a metal copper film on the surface of the resin substrate. Prior to carrying out the embodiment of the invention, it was attempted to conduct the treatments for forming a metal film as shown in FIG. 6 without conducting the etching treatment which involves high degree of danger and public hazard, while simplifying the steps of treatments in the method of forming the metal film.

[0040] In forming the metal film on the surface of the resin substrate as shown in FIG. 6, there are successively conducted a step S1 of a conditioning treatment, a step S2 of a pre-dipping treatment, a step S3 of a catalyzing treatment, a step S4 of an accelerating treatment, a step S5 of electroless copper plating, and a step S6 of electrolytic copper plating.

[0041] Here, the method of forming and treating the metal film shown in FIG. 6 is different from the conventional method with respect to the step of etching being omitted, that the pretreatment such as dewaxing of the surface of the resin substrate is conducted in the step S1 of conditioning treatment and, then, the pre-dipping treatment S2 is conducted for rendering the surface of the resin substrate to be hydrophilic and activated, followed by the step S3 of catalyzing treatment. The step S2 of pre-dipping treatment further works to promote the adsorption of complex of Pd and Sn that serves as plating nuclei.

[0042]FIGS. 7A to 7E schematically illustrate the steps of forming a metal film according to a procedure based on the treatment steps of the method of forming the metal film illustrated in FIG. 6. FIG. 7A corresponds to the step S1 of conditioning treatment, wherein a conditioning treatment solution 2 is brought into contact with the surface of the resin substrate 1 to effect the pre-treatment for the surface of the resin on which the plating is to be conducted.

[0043]FIG. 7B corresponds to the step S2 of pre-dipping treatment, wherein a pre-dipping treatment solution 3 is brought into contact with the surface of the resin substrate 1 to put the surface to the activating treatment. In FIGS. 7A and 7B, the conditioning treatment solution 2 or the pre-dipping treatment solution 3 is placed on the surface of the resin substrate 1, which is a schematic representation by giving attention to the treatment of the surface of the resin substrate 1. In a practical step of treatment, the resin substrate is treated by being immersed in each of the treatment solutions.

[0044] Next, FIG. 7C illustrates a state where the activation with Pd is finished on the surface of the resin substrate 1, i.e., where the step S3 of catalyzing treatment and the step S4 of accelerating treatment are conducted, and the plating nuclei 4 are precipitated. In the drawing, the plating nuclei 4 are shown by five black circles, for easy comprehension, on an enlarged scale. In practice, however, the plating nuclei 4 are very fine and are placed in a very many number.

[0045]FIG. 7D corresponds to the step S5 of electroless plating treatment, wherein the electroless copper plating treatment is conducted after the plating nuclei 4 are precipitated on the surface of the resin substrate 1. Due to the catalytic action of palladium, metal copper 5 is plated on the surfaces of the plating nuclei 4. The electroless plating is further continued, and metal copper is continuously plated on the whole surface of the resin substrate 1.

[0046]FIG. 7E corresponds to the step S6 of electrolytic copper plating treatment, wherein the electrolytic copper plating is effected after the metal copper 5 is electroless-plated on the whole surface of the resin substrate 1, thereby to form the metal film 6. Thus, the metal film that serves as a conducting layer is formed on the surface of the resin substrate 1 based on the method of forming the metal film illustrated in FIG. 6.

[0047] According to this method of forming and treating the metal film without the step of an etching treatment, however, the adhesion is weakened between the resin substrate and the metal film, and a sufficiently large peeling strength is not obtained. In order to increase the adhesion between the resin substrate and the metal film based on the method of forming a metal film, therefore, the inventors have attempted to improve the surface of the resin substrate by the irradiation with ultraviolet light. FIG. 8 illustrates a method of forming the metal film by the irradiation with ultraviolet light.

[0048] The method of forming the metal film illustrated in FIG. 8 is based on the steps of treatment of the method shown in FIG. 6, and includes the same treatment steps S1 to S6. In the method of forming the metal film of FIG. 8, a step S0 of treatment with ultraviolet light is conducted prior to the step S1 of conditioning treatment.

[0049] By irradiating the surface of the resin substrate with ultraviolet light, e.g., by using a monochromatic light of 172 nm emitted from a dielectric barrier discharge excimer lamp, it has been known that the surface of the polyimide resin can be activated. The effect of activation by the irradiation appears even when ultraviolet light has a long wavelength. However, it has been proved that the effect of activation increases with a decrease in the wavelength (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-57456). However, the effect of irradiation with ultraviolet light lasts for only a short period of time, and does not last up to the activation treatment with palladium or up to the treatment of electroless copper plating. Therefore, even though the effect of activation is obtained by the irradiation with ultraviolet rays, the adhesion is not improved between the resin substrate and the metal film.

[0050] In the method of forming and treating a metal film shown in FIG. 8, the step S0 of ultraviolet light irradiation treatment is solely conducted prior to the step S1 of conditioning treatment. It is, however, also attempted to irradiate the surface of the resin substrate with ultraviolet light simultaneously with the conditioning treatment or the catalyzing treatment of the surface. Namely, in the state of conditioning treatment shown in FIG. 7A or in the state of catalyzing treatment shown in FIG. 7B, the surface of the resin substrate is irradiated with ultraviolet light from the upper side of the resin substrate 1 through the conditioning treatment solution 2 or the catalyzing treatment solution 3.

[0051] However, no improvement is seen in the adhesion between the metal copper film 6 that is formed and the resin substrate 1 even when the metal copper film 6 is formed on the resin substrate 1 by irradiating the surface of the resin substrate 1 with ultraviolet light from the upper side of the resin substrate 1 through the conditioning treatment solution 2 or the catalyzing treatment solution 3.

[0052] Therefore, a cover glass plate is placed on the treatment solution such that the conditioning treatment solution 2 or the catalyzing treatment solution 3 assumes a small thickness of, for example, about 10 μm on the surface of the resin substrate 1. Then, the surface of the resin substrate 1 is irradiated with ultraviolet light having a wavelength of 172 nm through the cover glass plate to thereby form the metal copper film 6 on the resin substrate 1. The adhesion is greatly improved between the copper metal film 6 that is formed and the resin substrate 1, and there is obtained a peeling strength large enough for practical use.

[0053] The effect of improving the adhesion stems from that the component in the treatment solution and the resin molecules are chemically strongly bonded together since the resin and the treatment solution are made present together at a moment when the resin molecules on the surface are activated and are becoming highly reactive as the surface of the resin is irradiated with ultraviolet light through the treatment solution of a small thickness, and that the activated state is promoted and is lasting.

[0054] According to the method of forming and treating the metal film of this embodiment, therefore, ultraviolet light is irradiated at a timing while the treatment solution is present on the surface of the resin substrate in order to improve the adhesion between the resin substrate and the metal film and to enhance the activity by the irradiation with ultraviolet light, and the metal film is formed while the activity lasts.

[0055]FIG. 1 illustrates the steps of treatment of the method of forming a metal film according to a first embodiment of this invention. The method of forming and treating the metal film of the first embodiment of FIG. 1 deals with a case of irradiating ultraviolet light when the conditioning treatment solution exists while combining together the step S0 of ultraviolet light irradiation treatment and the step S1 of conditioning treatment shown in FIG. 8. Therefore, the steps of treatment of the method of forming the metal film according to the first embodiment is based on the method of forming and treating the metal film illustrated in FIG. 8, and the same steps of treatment are denoted by the same reference numerals.

[0056] In the method of forming and treating the metal film of the first embodiment, therefore, the step S0 of ultraviolet light irradiation treatment and the step S1 of conditioning treatment of FIG. 8 are replaced by the step S11 of conditioning and ultraviolet light irradiation treatment. However, the other steps such as the step S2 of pre-dipping treatment, the step S3 of catalyzing treatment, the step S4 of accelerating treatment, the step S5 of electroless copper plating treatment and the step S6 of electrolytic plating treatment, are the same as the treatments of the steps in the method of forming and treating the metal film of FIG. 6. Here, therefore, the step S2 and the subsequent steps are not described again.

[0057] Described below with reference to FIG. 2 is the step S11 of conditioning and ultraviolet light irradiation treatment, which is the feature of the method of forming and treating the metal film of the first embodiment. FIG. 2 is a schematic and partial representation on an enlarged scale. FIG. 2 illustrates a case of forming a metal copper film on an insulating resin layer that is laminated on, for example, a semiconductor device.

[0058] The layer of a conditioning treatment solution 2 is formed on the surface of the resin substrate 1 which is the insulating resin layer. In forming this layer, a cover glass plate 7 is placed on the conditioning treatment solution 2 so that the layer of the conditioning treatment solution 2 assumes a thickness of about 10 μm. The cover glass 7 is for uniformly and thinly forming the layer of the conditioning treatment solution 2 on the surface of the resin.

[0059] The surface of the resin substrate 1 is irradiated with ultraviolet light from the upper side of the cover glass plate 7. As ultraviolet light, there is used, for example, monochromatic light of 172 nm emitted from a dielectric barrier discharge excimer lamp. When the surface of the resin such as the polyimide resin is irradiated with ultraviolet light in the presence of the conditioning treatment solution 2, the surface is activated more strongly.

[0060] In the case of the first embodiment, the surface of the resin is more strongly activated, and the time for conducting the conditioning treatment may be shorter than the time of the step S1 of conditioning treatment in the method of forming and treating the metal film of FIG. 8. For the solution used for the conditioning treatment, it is not necessary to prepare a special solution, and an ordinary solution can be used as it is.

[0061] The step S11 of conditioning and ultraviolet light irradiation treatment shown in FIG. 2 is followed by an ordinary Pd activation treatment, electroless copper plating treatment and electrolytic copper plating treatment, successively, as shown in FIG. 1, and a metal film 6 of copper is formed on the resin substrate 1.

[0062] Next, FIG. 3 illustrates the steps of treatment of the method of forming the metal film according to a second embodiment of the invention. In the first embodiment, the step S0 of ultraviolet light irradiation treatment and the step S1 of conditioning treatment shown in FIG. 8 are combined together, and ultraviolet light is irradiated when the conditioning treatment solution is made present. In the method of forming and treating the metal film of the second embodiment, on the other hand, the step S0 of ultraviolet light irradiation treatment and the step S3 of catalyzing treatment shown in FIG. 8 are combined together, and ultraviolet light is irradiated when the catalyzing treatment solution is made present.

[0063] Therefore, the steps of treatment of the method of forming the metal film according to the second embodiment is based on the method of forming and treating the metal film illustrated in FIG. 8, and the same steps of treatment are denoted by the same reference numerals. In the method of forming and treating the metal film of the second embodiment, therefore, the step S0 of ultraviolet light irradiation treatment and the step S3 of catalyzing treatment of FIG. 8 are replaced by the step S31 of catalyzing and ultraviolet light irradiation treatment. However, the other steps such as the step S1 of conditioning treatment, the step S2 of pre-dipping treatment, the step S4 of accelerating treatment, the step S5 of electroless copper plating treatment and the step S6 of electrolytic plating treatment, are the same as the treatments of the steps in the method of forming and treating the metal film of FIG. 6. Here, therefore, the steps other than the step S31 are not described.

[0064] Described below with reference to FIG. 4 is the step S31 of catalyzing and ultraviolet light irradiation treatment, which is the feature of the method of forming and treating the metal film of the second embodiment. FIG. 4 is a schematic and partial representation on an enlarged scale like the case of FIG. 2. Illustrated here is a case of forming a metal copper film on an insulating resin layer that is laminated on, for example, a semiconductor device.

[0065] The layer of a catalyzing treatment solution 3 is formed on the surface of the resin substrate 1 which is the insulating resin layer. This layer is formed in the same manner as that of the case of FIG. 2; i.e., the catalyzing treatment solution 3 is simply used instead of the conditioning treatment solution 2. The cover glass 7 is placed on the catalyzing treatment solution 3 so that the layer of the catalyzing treatment solution 3 assumes a thickness of about 10 μm.

[0066] The surface of the resin substrate 1 is irradiated with ultraviolet light from the upper side of the cover glass plate 7. As ultraviolet light, there is used, for example, monochromatic light of 172 nm emitted from a dielectric barrier discharge excimer lamp. When the surface of the resin such as the polyimide resin is irradiated with ultraviolet light in the presence of the catalyzing treatment solution 3, the surface is activated more strongly.

[0067] In the case of the second embodiment, the surface of the resin is more strongly activated, and the time for conducting the catalyzing treatment may be shorter than the time of the step S3 of catalyzing treatment in the method of forming and treating the metal film of FIG. 8. There is used no particular treatment solution for the catalyzing treatment, and there is used the one that is usually used for the catalyzing treatment.

[0068] The step S31 of catalyzing and ultraviolet light irradiation treatment shown in FIG. 4 is followed by an ordinary Pd activation treatment, electroless copper plating treatment and electrolytic copper plating treatment, successively, as shown in FIG. 3, and a metal film 6 of copper is formed on the resin substrate 1.

[0069] In the method of forming and treating the metal film according to the first or second embodiment described above, the surface of the resin is irradiated with an ultraviolet ray in the presence of either one of the treatment solutions, i.e., in the presence of either the conditioning treatment solution or the catalyzing treatment solution. However, the surface of the resin can be activated more strongly even when the surface of the resin is irradiated with ultraviolet light in the presence of the above treatment solutions in both the step S1 of conditioning treatment and the step S3 of catalyzing treatment in the method of forming and treating the metal film illustrated in FIG. 6.

[0070]FIG. 5 illustrates a procedure of treatment of the method of forming and treating the metal film according to a third embodiment in which ultraviolet light is irradiated in each of the steps of treatment in the method of forming and treating the metal film illustrated in FIG. 6. In the method of forming and treating the metal film of the third embodiment illustrated in FIG. 5, the step S1 of conditioning treatment is replaced by the step S11 of conditioning and ultraviolet light irradiation treatment of the first embodiment, and the step S3 of catalyzing treatment is replaced by the step 31 of catalyzing and ultraviolet light irradiation treatment of the second embodiment.

[0071] In the method of forming and treating the metal film of the third embodiment, ultraviolet light is irradiated even in the step S5 of electroless copper plating treatment in the method of forming and treating the metal film of FIG. 6. In the step of electroless copper plating treatment, the electroless copper plating treatment itself is divided into two steps, i.e., into steps S51 and S54 of electroless copper plating treatment, and ultraviolet light is irradiated even between these electroless copper plating treatments.

[0072] After the plating treatment in the step S51, the surface of the resin substrate is irradiated with ultraviolet light in a state where there exist, in a mixed manner, a portion where the surface of the resin substrate is exposed and the metal copper film formed by the electroless copper plating in the step S52 of ultraviolet light irradiation treatment. The irradiation with ultraviolet light improves the adhesion between the surface of the resin and the metal copper precipitated by the electroless plating.

[0073] Even when the surface of the resin is irradiated with ultraviolet light during the step of electroless copper plating treatment, the adhesion of the metal copper film can be improved if a cover glass plate is placed on the surface of the resin and, then, if the surface of the resin is irradiated with ultraviolet light through the cover glass plate.

[0074] After the step S52 of ultraviolet light irradiation treatment, the step S53 of treatment with acid is effected to wash the surface of the metal copper that is precipitated and, then, the remaining electroless copper plating treatment is effected in the step S54 of electroless copper plating treatment. Then, the ordinary step S6 of electrolytic copper plating treatment is conducted to form a metal copper film on the metal copper that has been precipitated. When the metal copper film formed in the step S51 of treatment possesses a film thickness large enough to serve as a power feeding layer in the electrolytic copper plating treatment, then, the electroless copper plating treatment of the step S54 may be omitted.

[0075] As described above, the surface of the resin is irradiated with ultraviolet light in the presence of the treatment solution on the surface of the resin, whereby the surface of the resin is activated and becomes highly reactive due to the irradiation with ultraviolet light. At this moment, the component in the treatment solution is chemically and strongly bonded to the molecules on the surface of the resin, and the electroless copper plating enables metal copper to strongly adhere onto the surface of the resin.

[0076] According to the methods of forming and treating the metal film of the first to third embodiments, therefore, the adhesion between the resin substrate and the metal film formed on the surface of the resin substrate is improved accompanying an increase in the activity of the surface of the resin as it is irradiated with the ultraviolet ray in the presence of the treatment solution. Besides, upon simply contriving the timing for irradiating ultraviolet light, it is made possible to use a variety of treatment solutions, in various steps of treatment, by the method of forming a metal film on the surface of the resin substrate, without the need of providing any particular treatment.

[0077] In the methods of forming and treating the metal film of the first to third embodiments described above, the metal film is formed on the surface of the resin substrate through the step of plating nuclei formation treatment, step of electroless plating treatment, and step of electrolytic plating treatment. The adhesion between the resin substrate and the metal film formed on the surface of the resin substrate is improved accompanying an increase in the activity on the surface of the resin due to the effect of irradiating ultraviolet light in the presence of the treatment solution. Even when the metal film is formed on the resin substrate based solely upon the step of electroless plating treatment without effecting the step of electrolytic plating which is for forming part of the metal film, it is allowable to irradiate ultraviolet light in the presence of the treatment solution, and the adhesion is improved by the effect of the irradiation. Therefore, the method of forming and treating the metal film of this embodiment can be applied even when it is attempted to form the metal film relying simply upon the step of electroless plating treatment.

[0078] The foregoing description has dealt with the resin substrate that was chiefly made of the polyimide resin. Not being limited to the polyimide resin only, however, the method of forming the metal film of the embodiment can be further applied to fluorine-contained resins such as epoxy resin and polytetrafluoroethylene, as well as to acrylonitrile/butadiene/styrene resin and polycarbonate resin. If the surface of the resin can be improved and activated by the irradiation with ultraviolet light, there is no particular limitation on the material thereof and the adhesion of the metal film can be improved. When the metal film is to be formed on these resin substrates, the wavelength of ultraviolet light to be irradiated is suitably selected depending upon the materials, as a matter of course.

[0079] The method of forming the metal film according to this embodiment has dealt with the case of forming the metal copper film on the insulating resin layer that was laminated in the semiconductor device. As for the object of application, however, formation of the metal film is not limited to the semiconductor devices only, but can be applied to substrates using resins, to circuit wiring devices using a flexible sheet and to semiconductor packages using a built-up wiring substrate.

[0080] It is further allowable to use other metals such as nickel or chromium instead of copper, as a metal to be electrolytically plated for forming a metal film on the surface of the resin substrate when, for example, corrosion resistance is required.

[0081] Concretely described below is a method of forming and treating the metal film according to Examples. The description deals with a case of forming the metal copper film on the insulating resin layer made of the polyimide resin laminated in the semiconductor device. Prior to describing Examples, described below, first, for reference, Comparative Examples for demonstrating the effect of the method of forming and treating the metal film of the embodiment are described.

COMPARATIVE EXAMPLE 1

[0082] A metal film in Comparative Example 1 was formed in compliance with the steps of treatment of the method of forming and treating the metal film shown in FIG. 6.

[0083] In the step S1 of conditioning treatment, the surface of the polyimide resin was washed by using a conditioning treatment solution, CONDITIONER NEOPACT U, trade name, supplied by ATOTECH JAPAN K.K., containing a surfactant (containing 30 ml of the undiluted solution per liter of the treatment solution) at 45° C. for 5 minutes. Then, in the next step S2 of pre-dipping treatment, the surface of the polyimide resin was subjected to the treatment for adsorbing the plating nuclei by using a pre-dipping treatment solution, CATAPREP 404, trade name, supplied by SHIPLEY FAR EAST LTD., containing sodium chloride (NaCL) and sodium hydrogensulfate (NaHSO₄) (containing 200 ml of the undiluted solution per liter of the treatment solution) for one minute.

[0084] Next, in the step S3 of the catalyzing treatment, the catalyzing treatment was conducted by using a mixed treatment solution of the CATAPREP 404, trade name, supplied by SHIPLEY FAR EAST LTD. as a catalyst (containing 250 ml of the undiluted solution per liter of the treatment solution) and the CATAPOSIT 44, trade name, supplied by SHIPLEY FAR EAST LTD., containing palladium chloride (PdCl₂) and stannous chloride (containing 33 ml of the undiluted solution per liter of the treatment solution) at 45° C. for 5 minutes to adsorb a complex salt of Pd and Sn. Then, in the step S4 of accelerating treatment, the accelerating treatment was conducted by using the ACCELERATOR 19E, trade name, supplied by SHIPLEY FAR EAST LTD., containing borofluoric acid (HBF₄) (containing 40 ml of the undiluted solution per liter of the treatment solution) for 8 minutes so that palladium was precipitated as plating nuclei on the surface of the polyimide resin.

[0085] In the step S5 of electroless copper plating treatment, the electroless copper plating was conducted by using the CUPOSIT 328, trade name, supplied by SHIPLEY FAR EAST LTD., that is usually used with palladium as a catalyst, for 8 minutes, to precipitate metal copper to a thickness of 0.1 μm. Here, the CUPOSIT 328 comprises the CUPOSIT 328A containing copper sulfate (CuSO₄.5H₂O) (containing 120 ml of the undiluted solution per liter of the treatment solution), the CUPOSIT 328L containing Rochelle salt (C₄H₄O₆K.Na) and sodium hydroxide (NaOH) (containing 100 ml of the undiluted solution per liter of the treatment solution), and the CUPOSIT 328C containing formaldehyde (HCHO) (containing 15 ml of the undiluted solution per liter of the treatment solution).

[0086] Then, in the step S6 of electrolytic copper plating treatment, the electrolytic copper plating was effected by using the COPPER GLEAM 125, trade name, supplied by MELTEX INC. that is usually used while supplying an electric current in an amount of 2 A/dm² for 40 minutes to form a metal copper film having a thickness of 18 μm.

[0087] As described above, a metal film of copper was formed on an insulating layer using the polyimide resin as a substrate. The adhesion between the polyimide resin and the metal film was measured, and there was exhibited a peeling strength of about 100 gf/cm.

COMPARATIVE EXAMPLE 2

[0088] In Comparative Example 2, a metal film was formed in compliance with the steps of treatment of the method of forming and treating the metal film shown in FIG. 8. The method of forming and treating the metal film shown in FIG. 8 simply includes the additional step S0 of ultraviolet light irradiation treatment prior to effecting the steps of treatment in the method of forming and treating the metal film of FIG. 6.

[0089] Therefore, the steps S1 to S6 of treatment shown in FIG. 8 are quite the same as the steps S1 to S6 in Comparative Example 1, and are not described here. In the step S0 of ultraviolet light irradiation treatment, however, the surface of the polyimide resin was irradiated with ultraviolet light having a wavelength of 172 nm at an illumination of 13.44 mW/cm² for 120 seconds. The resin substrate irradiated with ultraviolet light was subjected to the treatments of the steps S1 to S6, successively.

[0090] Thus, a metal film of copper was formed on an insulating layer using the polyimide resin as a substrate. The adhesion between the polyimide resin and the metal film was measured, and there was exhibited a peeling strength of about 122 gf/cm.

EXAMPLE 1

[0091] Example 1 deals with a case of forming a metal film of copper on an insulating layer using the polyimide resin as a substrate in compliance with the method of forming and treating a metal film shown in FIG. 1. The method of forming and treating the metal film of FIG. 1 is based on the method of forming a metal film of FIG. 6, but in which the step S1 of conditioning treatment is replaced by the step S11 of conditioning and ultraviolet light irradiation treatment. Therefore, the step of forming the metal copper film on the surface of the polyimide resin substrate is based on the step of treatment in Comparative Example 1.

[0092] In Comparative Example 1, the surface of the polyimide resin was washed in the step S1 of conditioning treatment by using a conditioning treatment solution, CONDITIONER NEOPACT U, trade name, supplied by ATOTECH JAPAN K.K., containing a surfactant (containing 30 ml of the undiluted solution per liter of the treatment solution) at 45° C. for 5 minutes. In Example 1, however, the treatment was conducted for 120 seconds while irradiating ultraviolet light having a wavelength of 172 nm at an illumination of 13.44 mW/cm² from the upper side with the conditioning treatment solution 2, CONDITIONER NEOPACT U, trade name, supplied by ATOTECH JAPAN K.K. being interposed as a layer of a thickness of 10 μm between the cover glass plate 7 and the resin 1 as shown in FIG. 2.

[0093] After the treatment for activating the surface of the resin through the step S11 of treatment, there were conducted the steps of treatment according to the method of forming and treating the metal film of Comparative Example 1.

[0094] Thus, a metal film of copper was formed on an insulating layer using the polyimide resin as a substrate. The adhesion between the polyimide resin and the metal film was measured, and there was exhibited a peeling strength of not smaller than 570 gf/cm which was large enough for practical use.

EXAMPLE 2

[0095] Example 2 deals with a case of forming a metal film of copper on an insulating layer using the polyimide resin as a substrate in compliance with the method of forming and treating a metal film shown in FIG. 3. The method of forming and treating the metal film of FIG. 3 is based on the method of forming a metal film of FIG. 6, but in which the step S3 of catalyzing treatment is replaced by the step S31 of catalyzing and ultraviolet light irradiation treatment. Therefore, the step of forming the metal copper film on the surface of the polyimide resin substrate in Example 2 is conducted in the same manner as described above based on the step of treatment in Comparative Example 1.

[0096] In Comparative Example 1, the catalyzing treatment was conducted in the step S3 of catalyzing treatment by using a mixed treatment solution of CATAPREP 404, trade name, supplied by SHIPLEY FAR EAST LTD. (containing 250 ml of the undiluted solution per liter of the treatment solution) and CATAPOSIT 44, trade name, supplied by the same company containing palladium chloride (PdCl₂) and stannous chloride (SnCl₂) (containing 33 ml of the undiluted solution per liter of the treatment solution), as a catalyst, at 45° C. for 5 minutes to adsorb a complex of Pd and Sn. In Example 2, however, the treatment was conducted for 120 seconds by using, as a catalyst, a mixed solution of CATAPREP 404, trade name, and CATAPOSIT 44, trade name, both supplied by SHIPLEY FAR EAST LTD., while irradiating ultraviolet light having a wavelength of 172 nm at an illumination of 13.44 mW/cm² from the upper side with the catalyst being interposed as a layer of a thickness of 10 μm between the cover glass plate 7 and the resin 1 as shown in FIG. 4.

[0097] Thus, a metal film of copper was formed on an insulating layer using the polyimide resin as a substrate. The adhesion between the polyimide resin and the metal film was measured, and there was exhibited a peeling strength of not smaller than 700 gf/cm which was large enough for practical use.

EXAMPLE 3

[0098] Example 3 deals with a case of forming a metal film of copper on an insulating layer using the polyimide resin as a substrate in compliance with the method of forming and treating a metal film shown in FIG. 5. The method of forming and treating the metal film of FIG. 5 is based on the method of forming a metal film of FIG. 6, which, however, is based on a combination of the first embodiment and the second embodiment, and in which the step S1 of conditioning treatment is replaced by the step S11 of conditioning and ultraviolet light irradiation treatment, and the step S3 of catalyzing treatment is replaced by the step S31 of catalyzing and ultraviolet light irradiation treatment.

[0099] In the step S11 of conditioning and ultraviolet light irradiation treatment, therefore, the treatment was conducted for 120 seconds while irradiating ultraviolet light having a wavelength of 172 nm at an illumination of 13.44 mW/cm² from the upper side with the conditioning treatment solution 2, CONDITIONER NEOPACT U, trade name, supplied by ATOTECH JAPAN K.K. being interposed as a layer of a thickness of 10 μm between the cover glass plate 7 and the resin 1. In the step S31 of catalyzing and ultraviolet light irradiation treatment, on the other hand, the treatment was conducted for 120 seconds while irradiating ultraviolet light having a wavelength of 172 nm at an illumination of 13.44 mW/cm² from the upper side by using, as a catalyst, CATAPREP 404, trade name, and CATAPOSIT 44, trade name, both supplied by SHIPLEY FAR EAST LTD., and interposing the catalyst as a layer of a thickness of 10 μm between the cover glass plate 7 and the resin 1.

[0100] In the method of forming and treating a metal film of the third embodiment, further, the surface of the resin is irradiated with ultraviolet light even during the electroless copper plating treatment. In Comparative Example 1, the electroless copper plating treatment was conducted for 8 minutes by using the CUPOSIT 328, trade name, supplied by SHIPLEY FAR EAST LTD. In this Example 3, however, the electroless copper plating treatment was conducted being divided into two steps, and the surface of the resin substrate was irradiated with ultraviolet light during the electroless plating treatment in which there existed, in a mixed manner, a portion exposing the surface of the resin substrate and the metal copper film formed by the electroless copper plating. This helps improve the adhesion between the resin substrate and the metal copper due to electroless copper plating.

[0101] In the step S51 of electroless copper plating treatment, the polyimide resin substrate was immersed in an electroless copper plating treatment solution of the CUPOSIT 328, trade name, supplied by SHIPLEY FAR EAST LTD. while using palladium as a catalyst to conduct the electroless copper plating for 2 minutes. At this time, the thickness of the plated metal copper film was 0.025 μm. Then, the treated sample was taken out from the electroless copper plating treatment solution. In the step S52 of ultraviolet light treatment, the surface of the treated sample was irradiated with ultraviolet light having a wavelength of 172 nm at an illumination of 13.44 mW/cm² for 120 seconds.

[0102] Then, in the step S53 of treatment with acid, the sample was treated with a treatment solution containing 10% of sulfuric acid for 10 seconds to wash the surface of the metal copper on which the plating nuclei of palladium have been adhered as a catalyst. In the step S54 of the electroless copper plating treatment, the treated sample irradiated with ultraviolet light was immersed again in the above-mentioned electroless copper plating treatment solution in order to effect the additional electroless copper plating. The electroless copper plating treatment was conducted for 6 minutes to form a film of metal copper maintaining a thickness of 0.075 μm.

[0103] Then, as in Comparative Example 1, the electrolytic copper plating was conducted in the step S6 of electrolytic copper plating treatment by using the COPPER GLEAM 125, trade name, supplied by MELTEX INC., while flowing a current in an amount of 2 A/dm² for 40 minutes to form a metal copper film having a thickness of 18 μm.

[0104] Thus, a metal film of copper was formed on an insulating layer using the polyimide resin as a substrate. The adhesion between the polyimide resin and the metal film was measured, and there was exhibited a peeling strength of not less than 1050 gf/cm which was larger than the peeling strengths obtained in Examples 1 and 2.

[0105] According to the method of forming and treating a metal film as described in the above embodiments, the surface of the resin substrate is irradiated with ultraviolet light in the presence of a treatment solution. Therefore, metal copper is electroless-plated onto the surface while the surface is still being activated, and the adhesion between the surface of the resin and the metal film becomes greater than those of when the metal film is formed in Comparative Examples 1 and 2.

[0106] According to the method of forming and treating a metal film of the present invention as described above, the surface of the resin is irradiated with ultraviolet light in the presence of the treatment solution that is used in the steps of treatment. Therefore, the surface of the resin is activated by the irradiation with ultraviolet light, the electroless plating is executed while the effect of activation lasts, and improved adhesion is accomplished between the surface of the resin and the metal precipitated thereon due to the effect of ultraviolet light. Even without subjecting the surface of the resin to the etching treatment that is usually done, there is obtained the adhesion large enough for putting the metal plate to practical use.

[0107] Without using the etching treatment solution, further, it becomes easy to cope with the danger and public hazard, decreasing the burden of handling, treatment of drained solution and amount of working.

[0108] According to the method of forming and treating a metal film of the present invention, further, a time for irradiating ultraviolet light is contrived to be effected when there exists a treatment solution that is used in the steps of treatment in an attempt to improve the adhesion between the resin substrate and the metal film. Therefore, there is no need to provide a treatment agent for promoting the reforming of the surface of the resin substrate separately from the treatment solutions used in the ordinary steps of treatment. The customarily used treatment solution can be readily used, the number of the steps of treatment can be decreased by an amount equal to omitting the etching treatment, and the cost of treatment can be decreased, correspondingly.

[0109] As for reforming the surface of the resin substrate, no particular treatment apparatus must be installed but far the ultraviolet-light irradiator. Besides, the customarily used treatment solution can be used without increasing the cost related to the facilities, and the products can be inexpensively provided.

[0110] The method of forming a metal film on the surface of the resin substrate of the invention can be applied to forming a metal film on the laminated insulating layer in the semiconductor devices. Further, the metal film can be easily formed even on the resin substrate and the flexible sheet to inexpensively provide the products. 

What is claimed is:
 1. A method of forming and treating a metal film on the surface of a resin substrate by conducting the treatments according to a step of forming plating nuclei and a step of electroless plating, wherein said surface is irradiated with ultraviolet light through a treatment solution used prior to the step of electroless plating.
 2. A method of forming and treating a metal film according to claim 1, wherein said step of electrolytic plating is conducted after a step of electroless plating.
 3. A method of forming and treating a metal film according to claim 1 or 2, wherein said treatment solution is a conditioning treatment solution for pre-treating said surface prior to said step of forming plating nuclei.
 4. A method of forming and treating a metal film according to claim 3, wherein ultraviolet light is irradiated through said conditioning treatment solution and through a catalyzing treatment solution that is applied to the surface in said step of forming plating nuclei.
 5. A method of forming and treating a metal film according to claim 4, wherein said conditioning treatment solution or said catalyzing treatment solution is interposed in the form of a thin layer between a glass plate and said surface.
 6. A method of forming and treating a metal film according to claim 1 or 2, wherein said treatment solution is a catalyzing treatment solution applied to said surface in said step of forming plating nuclei.
 7. A method of forming and treating a metal film according to claim 6, wherein ultraviolet light is irradiated through a conditioning treatment solution that is used for treating said surface prior to said step of forming plating nuclei and through said catalyzing treatment solution.
 8. A method of forming and treating a metal film according to claim 7, wherein said conditioning treatment solution or said catalyzing treatment solution is interposed in the form of a thin layer between a glass plate and said surface.
 9. A semiconductor device having a metal film formed on an insulating resin layer by plating, said metal film being formed on the surface of said resin substrate through a step of forming plating nuclei and a step of electroless plating inclusive of irradiating said surface with ultraviolet light through a treatment solution that is used.
 10. A semiconductor device according to claim 9, wherein a step of electrolytic plating treatment is conducted after said step of electroless plating treatment.
 11. A semiconductor device according to claim 9 or 10, wherein said treatment solution is a conditioning treatment solution for pre-treating said surface prior to said step of forming plating nuclei.
 12. A semiconductor device according to claim 11, wherein ultraviolet light is irradiated through said conditioning treatment solution and through a catalyzing treatment solution that is applied to said surface in said step of forming plating nuclei.
 13. A semiconductor device according to claim 12, wherein said conditioning treatment solution or said catalyzing treatment solution is interposed in the form of a thin layer between a glass plate and said surface.
 14. A semiconductor device according to claim 9 or 10, wherein said treatment solution is a catalyzing treatment solution applied to said surface in said step of forming plating nuclei.
 15. A semiconductor device according to claim 14, wherein ultraviolet light is irradiated through a conditioning treatment solution that is used for treating said surface prior to said step of forming plating nuclei and through said catalyzing treatment solution.
 16. A semiconductor device according to claim 15, wherein said conditioning treatment solution or said catalyzing treatment solution is interposed in the form of a thin layer between a glass plate and said surface.
 17. A wiring board having a metal film formed on an insulating resin layer by plating, said metal film being formed on the surface of said resin substrate through a step of forming plating nuclei and a step of electroless plating inclusive of irradiating said surface with ultraviolet light through a treatment solution that is used.
 18. A wiring board according to claim 17, wherein a step of electrolytic plating treatment is conducted after said step of electroless plating treatment.
 19. A wiring board according to claim 17 or 18, wherein said treatment solution is a conditioning treatment solution for pre-treating said surface prior to said step of forming plating nuclei.
 20. A wiring board according to claim 19, wherein ultraviolet light is irradiated through said conditioning treatment solution and through a catalyzing treatment solution that is applied to said surface in said step of forming plating nuclei.
 21. A wiring board according to claim 20, wherein said conditioning treatment solution or said catalyzing treatment solution is interposed in the form of a thin layer between a glass plate and said surface.
 22. A wiring board according to claim 17 or 18, wherein said treatment solution is a catalyzing treatment solution applied to said surface in said step of forming plating nuclei.
 23. A wiring board according to claim 22, wherein ultraviolet light is irradiated through a conditioning treatment solution that is used for treating said surface prior to said step of forming plating nuclei and through said catalyzing treatment solution.
 24. A wiring board according to claim 23, wherein said conditioning treatment solution or said catalyzing treatment solution is interposed in the form of a thin layer between a glass plate and said surface. 